1. Field of the Invention
The present invention relates generally to workpiece inspection technologies and, more particularly, to pattern inspection techniques for inspecting a workpiece for defects, which is for use in semiconductor fabrication. This invention also relates to a method and apparatus for inspecting lithography masks for defects, which are for use in the manufacture of semiconductor devices and liquid crystal display (LCD) panels.
2. Related Art
In recent years, with the quest for higher integration and larger capacity of large-scale integrated (LSI) circuits, semiconductor devices are becoming narrower in circuit linewidth required. These semiconductor devices are fabricated by using an original or “master” plate with a circuit pattern formed thereon (also called a photomask or a reticle as will be generically referred to as a mask hereinafter) in a way such that the pattern is exposure-transferred by reduced projection exposure equipment, known as a stepper, onto a target wafer to thereby form thereon a circuit. Hence, for the manufacture of a mask to be used to transfer such ultrafine circuit pattern onto wafers, pattern photolithography equipment is used, which is capable of “drawing” microcircuit patterns. Such pattern exposure equipment is also employable in some cases to directly draw or “image” a circuit pattern onto wafers. As for the pattern exposure equipment, an attempt is made to develop exposure tools using an electron beam or laser beam.
Improving manufacturing yields is inevitable for the microfabrication of LSI chips which entail increased production costs. Currently, circuit patterns of LSIs, such as 1-gigabit class dynamic random access memories (DRAMs), are becoming on the order of nanometers (nm), rather than submicron order. One major factor for reducing yields must be pattern defects of a mask as used when an ultrafine pattern is exposed and transferred onto semiconductor wafers by photolithography techniques. As LSI patterns to be formed on semiconductor wafers are further miniaturized in recent years, the size dimensions that must be detected as pattern defects became much smaller than ever before. Thus, a need is felt to achieve further increased accuracy of the pattern inspection apparatus operable to inspect the LSI fabrication-used pattern-transfer mask for defects.
Incidentally, with recent advances in multimedia technologies, LCD panels are becoming larger in substrate size, up to 500 mm×600 mm or more, and finer in pattern of thin film transistors (TFTs) as formed on liquid crystal substrates. This larger/finer trend requires an ability to inspect ultrasmall pattern defects in a wide range. For this reason, it is an urgent challenge to develop an advanced workpiece inspection apparatus capable of efficiently inspecting defects of photomasks in a short time period, which are for use in the manufacture of such large-area LCD patterns and large-screen LCD panels.
Here, in currently available pattern inspection tools, it is known to perform inspection by comparing the optical image of a pattern on a workpiece, such as a lithography mask or else, which image is sensed by using a magnifying optical system at a specified magnification, to either design data or a sensed optical image of an identical pattern on the workpiece. This approach is disclosed, for example, in JP-A-8-76359.
Examples of pattern inspection methodology include a “die to die” inspection method and a “die to database” inspection method. The die-to-die inspection is for comparing together optical images as sensed from identical pattern elements at different locations on the same mask. The die-to-database inspection is usually performed using an exposure device for drawing or “imaging” a pattern on a mask and an inspection device. Typically this inspection has the steps of receiving computer-aided design (CAD) data indicative of a designed pattern, converting the CAD data to pattern draw data having a format appropriate for data input to the imaging device, inputting the converted data to the inspection device, causing it to generate a reference image based thereon, receiving measured data indicative of the optical image of a pattern under testing as obtained by pickup of this pattern, and then comparing the optical image to the reference image to thereby inspect the under-test pattern for defects. The inspection method for use in such apparatus, the workpiece is mounted on a stage, which moves to permit light rays to scan a surface of the workpiece for execution of the intended inspection. A light source and its associated illumination optical lens assembly are used to emit and guide the light to fall onto the workpiece. The light that passed through the workpiece or reflected therefrom travels via the optics to enter a sensor so that a focussed optical image is formed thereon. This optical image is sensed by the sensor and then converted to electrical measurement data, which will be sent to a comparator circuit. After position-alignment between images, the comparator circuit compares the measured data to reference image data in accordance with an adequate algorithm. If these fail to be matched, then determine that pattern defects are present.
As previously stated, with the growing quest for highly miniaturized workpiece patterns, a need is felt to accurately detect ultrafine defects, which are small in size enough to be “buried” in pixel position deviations, image expansion/shrink, swell and sensing noises of those images to be compared together. To meet the need, a high-reliability comparison inspection technique is desired.
A known approach to detecting ultrafine defects residing at pattern edges in the case of comparing an optical image to reference image is to extract contour lines for comparison. Examples of it are disclosed, for example, in JP-A-11-132743 and JP-A-2002-203233. Other known approaches include the use of a method of detecting defects while combining together a plurality of predetermined filters in a way pursuant to specific shapes, such as horizontal and/or vertical edge shapes.
Unfortunately, the prior art approaches are encountered with the difficulty in accurately extracting, with increased stability, contour lines from optical images which can contain various kinds of noises. Another difficulty lies in the lack of an ability to offer detectability against gently sloped edge portions having hardly specifiable contour lines and mere “painted” portions within patterns, which come with inherently unidentifiable contour lines. Additionally in the case of using “inflexible” filters designed for exclusive use with predefined shapes only, inspection-applicable pattern shapes must be limited. Although in this filtering scheme the so-called differential (difference) computation is often employed, it remains difficult to obtain an accurate differential (difference) value from those edges having complicated profile shapes.
As apparent from the foregoing, the prior known schemes for comparing an optical image to a reference image using a filter which is exclusively fitted to contour lines and/or specific shapes are encountered with problems as to the lack of supportabilities to comparison at those edges with various angles and edges having complicated profiles. Another problem faced with the prior art lies in the difficulty in handling mere “painted” portions inside of inherently contour-unidentifiable patterns and gently sloped edges with their contour lines being rarely identifiable.